There is a spectrum of involvement in those with the FMR1 premutation that ranges from developmental problems in childhood and psychopathology in adulthood to neurodegeneration in aging; the latter includes the Fragile X-associated Tremor Ataxia Syndrome (FXTAS) that we have identified with our Australian collaborators. However, some individuals with the premutation have no overt clinical involvement, whereas others experience mild or severe deficits in childhood, adulthood or aging. Typically, the developmental problems are thought to be related to FMRP deficits, whereas the aging problems to RNA toxicity secondary to elevated FMR1-mRNA. We will assess the influence of specific domains of molecular/cellular dysfunction, including mitochondrial dysfunction, inter-tissue (lymphocyte-fibroblast) mosaicism, occult methylation, and increases of disease-associated antisense FMR1 (ASFM1) isoforms on the nature and degree of the clinical phenotypes in both children and adults who are carriers of premutation alleles. Specific clinical outcome measures will be used that can document mild to severe involvement in the neurocognitive, motor and psychiatric realms. We will assess 189 patients (10 to 30 years) including 63 carriers who are affected, 63 who have normal development (asymptomatic) and 63 normal controls matched on age and sex to define the relationship between the severity of premutation developmental problems, including cognitive, motor and social deficits, and the presence of specific molecular abnormalities (Specific Aim 1). We will extend these studies to characterize the nature and severity of molecular dysfunction in adults with the premutation who have psychiatric and/or neurological dysfunction but do not meet clinical criteria for FXTAS (Specific Aim 2); we will recruit 201 patients (30 to 65y), divided between carriers who are affected with neurological or psychiatric problems, asymptomatic carriers, and controls matched for age and sex. We will determine whether the spectrum of clinical involvement in premutation adults is paralleled by severity of molecular dysregulation or whether the molecular features of FXTAS are distinct, and whether early biomarkers can be identified to use in future treatment endeavors. Lastly we will utilize sensitive neuroimaging techniques (diffusion tensor imaging; DTI) to assess involvement in adults with the premutation to determine the degree of concordance between the loss of integrity of fiber tracts in white matter and the severity of the mitochondrial dysfunction in adul carriers with psychiatric and/or neurological problems compared to unaffected carriers and controls (Specific Aim 3). This work will continue to include the Australia team, who will carry ou the occult methylation studies on blood and fibroblasts for all patients seen at both sites, in addition to limited recruitment for Aims 1 and 2. Our studies will provide biomarkers of early involvement and lay the basis for treatments such as mitochondrial based interventions, targeted treatment for low FMRP, or medications that block RNA toxicity early on.